The invention concerns a stopper rod for regulating the flow of a liquid, having a porous part fed with gas.
Stopper rods are frequently used in industry for opening and closing an orifice of a receptacle containing a liquid such as molten steel in a metallurgical vessel. By movement of the stopper rod from the orifice of the receptacle, the flow rate of the liquid is regulated. In some cases, these flow regulating stopper rods have appropriate internal channels and /or porous portions which make it possible to blow a treating gas into the liquid contained in the receptacle. Thus, the use of a stopper rod for controlling the flow of a molten steel emerging from a tundish into a water-cooled, continuous-casting mold is well known. In addition, it is conventional practice to introduce an inert gas, generally argon, into the molten metal through the stopper rod. The purpose of the argon is to eliminate unwanted inclusions contained in the molten steel. Another purpose is to reduce the deposits of alumina that occur in the casting elements, particularly when casting aluminum killed steel. Finally, the injection of argon makes it possible to avoid the development of a vacuum inside of the casting elements. Such a vacuum is capable of causing an aspiration of air through the porous refractory casting elements which, in turn, causes harmful oxidation of the molten metal.
According to one known technique, the inert gas is injected by means of an axial channel that passes through the stopper rod and exits at the end thereof. A further known stopper rod has a separate, porous stopper or plug sealed in the refractory material at the end of the axial channel of the stopper rod for emitting the inert gas to the molten metal. In the case of the first-mentioned technique, the hole at the end of the stopper rod has a substantial diameter, on the order of 2-3 mm. Consequently, a back flow of molten metal can occur through this orifice in the case where the pressure of the inert gas is interrupted for any reason. Furthermore, the gas injection is localized at one point and induces large bubbles that are less effective for eliminating the impurities contained in the metal. The second solution mentioned above makes it possible to produce small bubbles distributed on the surface of the porous stopper, however, there is the risk of unsealing of this stopper which leads to a back flow of molten metal within the axial channel of the stopper rod.
U.S. Pat. No. 4,791,978 to Mark K. Fishler, and owned by assignee of the present invention, discloses a stopper rod having a porous nose isostatically co-pressed at the same time as the body. The porous nose has a composition similar to that of the body, but its permeability is much higher. The copressing makes it possible to avoid the risk of losing the porous nose.
Nevertheless, in the stopper rod according to U.S. Pat. No. 4,791,978, the internal surface of the end of the axial channel of the stopper rod is relatively small. In addition, the thickness of the porous material which must be traversed by the inert gas is substantial, e.g., on the order of 40 mm, for a stopper rod of current dimensions. These characteristics lead to a limitation of the inert gas flow rate obtained at elevated temperatures. Thus, the maximum flow rate of argon obtained at 1500.degree. C. is about 6-8 Nl/min. for a molten metal counterpressure of 2.8 bar. This flow rate is insufficient in some cases and also the relatively substantial counterpressure that is necessary is dangerous for the axial channel, the connection of the stopper rod and the gas feed piping. The principal risk is the bursting of the nose during the casting, the catastrophic consequence of which would be loss of control of the molten metal flow. The second problem is a high risk of leakage in the gas connections, leading to the inefficacy of gas flow through the porous nose.
Given that a high resistance to erosion by steel is necessary, all the attempts made to increase the permeability of the porous material constituting the nose failed because an increase in the number of pores and/or an increase in their size result in an inacceptable reduction in the erosion resistance of the porous material.
The present invention provides a remedy to these shortcomings of the prior art.
An object of the invention is to create a stopper rod for regulating the flow of a liquid that preserves the advantages of stopper rods of the prior art, while permitting an increase in the flow rate of gas. The improved flow rate is obtained at a lower inert gas pressure than heretofore possible.